The service life of polyamides is dependent on (1) the raw materials used in the manufacturing process, (2) the additives used in stabilizing the polymer against damage brought about by melt-processing and (3) environmental weathering factors. A serious drawback to providing stabilization systems for polyamides lies in the fact that the maximum concentration of many additives is limited. Polyamides are heretofore regarded as poor in solvating additives, especially nonpolar additives having relatively low molecular weight in relation to the polyamide polymer. The aesthetics of an article such as an injection molding can be affected such as by migration, blooming or plate-out of the additives. On the other hand, manufacturers of polyamide-based moldings and extrusions strive to include low concentrations of additives for economic reasons.
Oxalic acid dianilides, formed on condensing the acid, partial ester or di-ester with one or two moles of aniline derivatives are known light stabilizers. Unsymmetrical oxalic acid dianilides are described, for example in U.S. Pat. No. 4,003,875. U.S. Pat. No. 4,544,691 discloses stabilizer compounds containing UV-absorbing derivatives of oxanilide, benzalmalonates, .alpha.-cyanocinnamates and o-hydroxyphenylbenzotriazoles, having a reactive phenolic group(s). These additives are suggested for use with polymer coating materials.
Polyamide polymers are characterized by high tensile strength, modulus and abrasion resistance, however polyamides require stabilization against thermo-oxidative and photodegradation for demanding use applications. Therefore, further improvement in the stabilization systems for these materials is presently desired in such end uses as fibers, and especially in relatively high temperature processing of injection moldings. The polyamide structure contains labile hydrogens and exert reductive or basic properties which are problematic from the standpoint of colorants. For example, in polyamide, organic red dyes undergo significant loss of shade upon thermal processing at the typical melt processing temperatures for polyamide. It would be desirable to lessen the tendency of polyamide to exert this influence on dyes without sacrificing the desired physical properties of polyamides.
The evolution of polyamide stabilization systems has advanced from earlier recommended hindered phenolic types, for example Irganox.RTM. 1098, to a combination of an antioxidant and a benzotriazole, such as the combination of Irganox.RTM. 1098 with Tinuvin.RTM. 234. Lightfast and photochemically stable dyed polyamide fibers containing oxalic acid di-aryl amides with a copper complex are known, for example from U.S. Pat. No. 5,338,319. These are applied in aqueous dye baths. In contrast, the present invention is directed to additives which are dispersed within the polyamide polymer by melt-processing at high temperatures above the glass transition, as in extrudates or moldings.
A stabilization system for polyamides has been found which exhibits unexpected synergisitic improvements. There is provided a modification of polyamide at low effective additive concentrations with improved technical and economic effects.